Switched mode power supplies are known in modem electronics as an efficient means of dc-to-dc conversion. These supplies provide different power supply characteristics. For example, a boost converter provides a dc-to-dc conversion in which the output voltage is higher than the input voltage. A buck converter, on the other hand, provides an output voltage that is lower than the input voltage. The buck-boost converter converts a positive input voltage into a negative output voltage.
These supplies operate from low impedance input power sources, such as pure voltage sources or voltage sources with low impedance in series. Such switched mode power supplies can be referred to as voltage-sourced power supplies or voltage-sourced converters. Such supplies may include a switch operated at a high rate, with each switch closing operation serving to store energy in an inductor, and each switch opening operation effecting a transfer of the stored energy from the inductor to a load connected to a regulator output. When applied to a low impedance voltage source, wherein the source voltage remains substantially constant over a range of source current, the duty cycle of the switch, i.e. the portion of time within each switching cycle that the switch is closed, or on, is varied in direct relation to the desired change in the regulator output voltage. That is, in response to an undesirably low regulator output voltage, e.g. due to an additional load demand or a decrease in source voltage, the duty cycle is initially increased in order to increase the output voltage. Correspondingly, the response to an excessive regulator output voltage is a decrease in the duty cycle, which is effective to decrease the output voltage.
One known topology for voltage-sourced switched-mode power supplies is the buck or step down converter, which provides an output voltage that is lower than the input voltage. As shown in FIG. 1, a simple buck converter 100 comprises input terminals 109 and 110 and output terminals 111 and 112 where terminals 110 and 112 are connected to ground. An input filter such as capacitor 102 is connected across input terminals 109 and 110; a shunt switch 103 is connected between input terminal 109 and output terminal 111. An energy storage element such as an inductor 104 is connected in series between the switch 103 and output terminal 111. A unidirectional conducting device, such as diode 108, is connected between ground and a point between switch 103 and inductor 104. An output filter such as capacitor 105 is connected across output terminals 111 and 112. A control device 106 is connected to both output terminal 111 and the control terminal for switch 103. A voltage source 101, with its small, effective, in series resistance, is connected across terminals 109 and 110 to provide power to the power converter. A load 107 is attached across terminals 109 and 110.
The control device 106 senses the voltage at output terminal 111, and controls the duty cycle of switch 103, i.e. the switch “on” time relative to the switch “off” time, to achieve the desired regulator output voltage at output terminal 111. When switch 103 is turned on, current flows through inductor 104 and across the load 107 such that the output voltage is equal to the input voltage. The current through inductor 104 causes energy to be stored in the inductor. When switch 103 is turned off, inductor 104 acts as a source, and current continues to flow across load 107. The rate at which the switch is operated is controlled by control device 106 in order to regulate the output voltage. The transfer function for this buck converter, i.e., how the output voltage is related to the input voltage is: V.sub.o=V.sub.i*DU where
Vo is the output voltage at terminal 111; Vi is the input voltage at terminal 109; and DU is the duty cycle with DU=T.sub.on/T where T is the period of the switching frequency and Ton is the time within each cycle that the switch is on or closed. For example, a 50% duty cycle, where the switch is on half the time, will result in the output voltage being half the input voltage.
The feedback built into the system through control device 106 controls the duty cycle in response to load voltage requirements. When the output load increases, the output voltage drops. This drop is detected by control device 106, which causes the duty cycle to increase, increasing the output voltage.
Another known topology for voltage-sourced switched-mode power supplies is the boost or step up converter shown in FIG. 2, which provides an output voltage that is higher than the input voltage. In FIG. 2, the output voltage at terminal 211 of boost converter 200 is controlled by switch 203, which responds to feedback from the output voltage at terminal 211. The transfer function for the voltage-sourced boost converter is: V.sub.o=V.sub.i/(1−DU).
In these known converters, if the input voltage source were replaced with a current source, i.e., a pure current source or a voltage source with a very high output impedance in series, the feedback loop would decrease the input power, since the input voltage is decreasing in response to an increase in the duty cycle—the opposite of what is required. Such a power supply can be referred to as a current-sourced power supply or a current-sourced converter.
High source impedance, therefore, makes the known voltage-sourced converters unstable. Assume, for example, zero source impedance. When the output load of a voltage-sourced converter increases, the only effect is the output voltage dropping slightly (due to increased losses), causing the feedback loop to increase the duty cycle. However, adding some input resistance causes the input voltage to drop further due to the voltage drop across this series resistance, requiring an additional increase in duty cycle. This causes the input current to increase, dropping the input voltage more, and increasing the duty cycle again. For some value of input resistance, this mechanism is unbounded, and leads to zero input voltage and maximum duty cycle.
Dr. R. D. Middlebrook recognized this condition and established the Middlebrook criterion for power supply design. The Middlebrook criterion requires that the source output impedance, which is often dominated by the impedance of the power supply's input filter, be much lower than the power supply input impedance, which is a function of the impedance of the power supply's output filter and loop response.
Certain applications, however, result in a high impedance input source. For example, in applications having long input cables, such as telecommunications systems e.g., power to telephones in the public telephone system; data collecting with instruments powered at the end of long cables, including, for example, borehole electronics placed at the end of a long cable fed into a borehole for sensing and measurement purposes); and systems deployed along long undersea cables, the cables provide a high impedance input to any power supplies at the remote end of the cable. For these applications, shunt or linear regulators are currently used instead of switched-mode power supplies. The shunt regulator, however, is less efficient than a switched-mode power supply because it has the undesirable property of drawing a constant power from the source, no matter what the load is. At typical or minimum loads, therefore, the shunt regulator efficiency is very poor.
Other applications that result in a high impedance input source might include, for example, medical applications where power is transmitted through the body to a device located in the body. In such applications the air and tissue through which the power must be transmitted have a high impedance so that the internal power supply will see an input source with a high impedance. This might also be the case in other applications where power is transmitted wirelessly, such as providing power to a passive smart card. In such an example the high impedance of the air through which the power is transmitted results in the power supply of the device seeing a high impedance input source.
What is needed is a switched-mode power supply that exhibits stable operations with a high impedance input source. As disclosed herein, known converters can be reconfigured to accommodate a high impedance source by arranging the components so as to reverse the phase of the feedback so that the duty cycle decreases instead of increases in response to a decrease in the output voltage.